Quantum turbulence visualized by particle dynamics

The Lagrangian dynamics of micrometer-sized solid particles of hydrogen and deuterium is investigated in thermal counterflow of superfluid He-4 at length scales l(exp) straddling about two orders of magnitude across the average distance l between quantized vortices by using the particle tracking velocimetry technique. The normalized probability distribution functions of the particle velocities and accelerations change from the shapes typical of quantum turbulence, characterized by power-law tails, at length scales l(exp) less than or similar to l, to forms similar to those obtained in classical turbulent flows, at l(exp) greater than or similar to l, although the power-law behavior of the acceleration distribution tails is less clear than that observed for the particle velocities. Moreover, the acceleration distribution follows a nearly log-normal, classical-like shape, at l less than or similar to l(exp) less than or similar to L-int, where L-int denotes the integral length scale, providing thus, within the just defined inertial range, experimental evidence of the existence of classical-like, macroscopic vortical structures in thermal counterflow of superfluid He-4, which is traditionally regarded as a quantum flow with no obvious classical analog. Additionally, we report our observations of the added mass effect in quantum turbulence and discuss them in the framework of a developed model of particle dynamics.